Interpreting Number Ranges in Python

Question

In a Pylons webapp, I need to take a string such as "<3, 45, 46, 48-51, 77" and create a list of ints (which are actually IDs of objects) to search on.

Any suggestions on ways to do this? I'm new to Python, and I haven't found anything out there that helps with this kind of thing.

The list would be: [1, 2, 3, 45, 46, 48, 49, 50, 51, 77]

Answer 1

Use parseIntSet from here

I also like the pyparsing implementation in the comments at the end.

The parseIntSet has been modified here to handle "<3"-type entries and to only spit out the invalid strings if there are any.

#! /usr/local/bin/python
import sys
import os

# return a set of selected values when a string in the form:
# 1-4,6
# would return:
# 1,2,3,4,6
# as expected...

def parseIntSet(nputstr=""):
    selection = set()
    invalid = set()
    # tokens are comma seperated values
    tokens = [x.strip() for x in nputstr.split(',')]
    for i in tokens:
        if len(i) > 0:
            if i[:1] == "<":
                i = "1-%s"%(i[1:])
        try:
            # typically tokens are plain old integers
            selection.add(int(i))
        except:
            # if not, then it might be a range
            try:
                token = [int(k.strip()) for k in i.split('-')]
                if len(token) > 1:
                    token.sort()
                    # we have items seperated by a dash
                    # try to build a valid range
                    first = token[0]
                    last = token[len(token)-1]
                    for x in range(first, last+1):
                        selection.add(x)
            except:
                # not an int and not a range...
                invalid.add(i)
    # Report invalid tokens before returning valid selection
    if len(invalid) > 0:
        print "Invalid set: " + str(invalid)
    return selection
# end parseIntSet

print 'Generate a list of selected items!'
nputstr = raw_input('Enter a list of items: ')

selection = parseIntSet(nputstr)
print 'Your selection is: '
print str(selection)

And here's the output from the sample run:

$ python qq.py
Generate a list of selected items!
Enter a list of items: <3, 45, 46, 48-51, 77
Your selection is:
set([1, 2, 3, 45, 46, 77, 48, 49, 50, 51])

Answer 2

I've created a version of @vartec's solution which I feel is more readable:

def _parse_range(numbers: str):
    for x in numbers.split(','):
        x = x.strip()
        if x.isdigit():
            yield int(x)
        elif x[0] == '<':
            yield from range(0, int(x[1:]))
        elif '-' in x:
            xr = x.split('-')
            yield from range(int(xr[0].strip()), int(xr[1].strip())+1)
        else:
            raise ValueError(f"Unknown range specified: {x}")

In the process, the function became a generator :)